(NY) A 4-month old quarter horse foal was found down in the field with markedly distended and firm epaxial and gluteal muscles. Initial bloodwork and urinalysis revealed marked hyperglycemia, significant increases in creatine kinase (CK) and aspartate transaminase (AST), and brown urine. The foal arrested and died despite veterinary intervention.

A field necropsy was performed and gross findings included firm and pale skeletal muscle and emphysematous lungs. Fresh skeletal muscle was submitted to the Equine Neuromuscular Laboratory at Michigan State University which revealed depletion of glycogen, and was suggestive of either malignant hyperthermia or myosin heavy chain myopathy (MYHM).  Histopathology performed on fixed tissues submitted to the Cornell AHDC Anatomic Pathology Service revealed acute bacterial pneumonia, multi-organ thrombi, and changes in skeletal muscles that were compatible with those reported by MSU. Hair samples sent to the University of California, Davis for the Quarter Horse & Related Breeds Disease PCR Panel (5-panel plus) confirmed the presence of 1 copy of MYH1 mutation in this foal, indicating this foal was heterozygous for MYHM.

MYHM is an autosomal codominant genetic disorder that occurs in quarter horses and related breeds, and presents as two equine syndromes:  non-exertional rhabdomyolysis or immune-mediated myositis (IMM). The foal in this case demonstrated clinical signs consistent with non-exertional rhabdomyolysis, which affects young horses.  Clinical signs include swollen muscles, gait stiffness, muscle pain, and potential recumbency and simultaneous elevations of muscle enzymes (AST and CK). Some horses who recover from non-exertional rhabdomyolysis develop acute muscle atrophy, which is also typical of IMM. IMM occurs in young horses or adults, when the immune system attacks the skeletal muscle fibers affected by the genetic MYH1 mutation. Horses with IMM present with clinical signs of rapid, symmetric atrophy of lumbar and gluteal muscles. Development of IMM can be triggered by exposure to Streptococcus equi subsp. equi, or other infectious diseases, and has occurred in horses following vaccination. For more information on the diagnosis, treatment and prevention of MYHM-related syndromes in horses, please visit the Equine Neuromuscular Laboratory at Michigan State University website.

Valberg, Stephanie J. "Genetics of Equine Muscle Disease." Veterinary Clinics: Equine Practice 36.2 (2020): 353-378.